FIG. 1 is a block diagram illustrating selected components of an information storage system (disk drive) 110 according to of the prior art. Disk drives have one or more disks 111 on which ferromagnetic thin materials are deposited. The disk drive includes data recording disk 111, pivoting actuator arm 113, and slider 112 that includes a read head and a write head. The functional blocks include servo system 90, read/write electronics 114, interface electronics 115, controller electronics 116, microprocessor 117, and RAM 118. A disk drive can include multiple disks stacked on a hub that is rotated by a disk motor, with a separate slider for each surface of each disk. The term servo wedge 120 will be used to mean the contiguous set of servo fields extending from ID to OD on the disk.
Disk 111 will typically have multiple servo wedges 120 arranged radially around the disk, but only two are shown for simplicity. Information recorded on the disks is generally organized in concentric tracks or, alternatively, the tracks can be arrange in a plurality of spiral tracks. (For a description of spiral tracks see, for example, U.S. Pat. No. 7,113,362 Lee, et al. Sep. 26, 2006.) In embodiments either of these tracks organizations can be used, and the term “tracks” will be used generically to include these any other similar forms of arrangement.
As part of the manufacturing process permanent servo information is recorded on the disks that provides information to the system about the position of the heads when the disks are rotating during operation. The servo identifier (SID) data on the disk provides several fundamental functions and is conventionally arranged in four distinct fields in each of the plurality of servo sectors angularly spaced around the disk. First, the servo data supplies a timing mark (known as the Servo Track Mark (STM) or equivalently Servo Address Mark (SAM)) which is used to synchronize data within the servo fields, and also provides timing information for write and read operations in the user data portions of the track. Second, the servo area supplies a 10-30 bit digital field, which provides a coarse track-ID (TID) number and additional information to identify the physical servo sector number. The TID is typically written in Gray code as the presence or absence of recorded dibits. During seek operations, when the head is moving across tracks, the head can typically only read a portion of the Gray-code in each TID. The Gray-code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek.
Finally, the SID field supplies a position error field, which provides the fractional-track Position Error Signal (PES). Auxiliary functions, such as amplitude measurement control or repeatable run-out (RRO) fields are sometimes also used. During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track. The typical PES pattern includes a burst pattern in which the bursts are identical sets of high frequency magnetic flux transitions. Unlike the track-ID (TID) field number, the PES bursts do not encode numerical information. In contrast to the TID, it is the position of the bursts that provide information on where the head is relative to the centerline of a track. In a quad-burst PES, the pattern is repeated for each set of four tracks, so only local information is provided. Each servo wedge has four (A, B, C, D) sequential slots reserved for PES bursts, but each track has a centered PES burst in only one of the four slots. Each burst is centered on a selected track, but its width extends to the centerline of adjacent tracks. Thus, when the head is centered over a selected track, it will detect the strongest signal from a burst centered on the selected track, but it will also detect a weaker signal from bursts on the adjacent tracks. For example, when the head is centered over a track with a burst in the A-position, it might also detect a subsequent weak B-burst on the adjacent track on the right and then a weak D-burst from the adjacent track on the left. When the head passes over the PES pattern, the bursts that are within range generate an analog signal (waveform) that indicates the position of the head over the disk and is used as feedback to adjust the position of the head. Variations of the standard quad-burst pattern described above include use of two conventional, single frequency, quad burst servo patterns interspersed with dual frequency, dual burst servo patterns as described by Serrano, et al. in U.S. Pat. No. 6,078,445.
Each of these servo functions typically consumes a relatively independent portion of the servo wedge in prior art servo systems. The overhead on the disk to support these functions is a large factor in the drive's format efficiency. Typically, the servo fields can consume a significant portion of the recording surface of the disk and are an attractive target for reduction.
U.S. Pat. No. 6,967,808 to Bandic, et al. describes a servo pattern having pseudo-random binary sequences for the servo information used to control the position of the recording head. The automatic gain control (AGC), servo timing mark (STM) and PES fields in the prior art are replaced by a pseudo-random binary sequence (PRBS) field. The TID field, which is not included in the PRBS, is encoded twice using non-return to zero (NRZ) encoding, which results in a smaller field and is more efficient than the prior art dibit encoding method used for Gray codes. The PRBS fields are also written using NRZ encoding.
Related prior art includes U.S. Pat. No. 7,193,800 to Coker et al. which describes the use of particular pseudo-noise (PN) or pseudo-random sequence fields for the purpose of PES and rudimentary TID detection. The AGC, STM, TID, and PES fields in the prior art are replaced by a pair of pseudo-random binary sequence (PRBS) fields.
Published US patent application 20090168227 by Blaum, et al. describes a method of distributed track-ID in which first and second portions of a track-ID are physically separated in a disk sector. Each of the portions of the track-ID is encoded using a Gray code.
The Integrated Servo concepts which are referenced herein are described in published U.S. patent applications:    20110149434 by Coker, et al. (pub. Jun. 23, 2011), Ser. No. 12/653,874, filed Dec. 18, 2009;    20110149433 by Coker, et al. (pub. Jun. 23, 2011), Ser. No. 12/653,863, filed Dec. 18, 2009    20110149432 by Coker, et al. (pub. Jun. 23, 2011), Ser. No. 12/653,862, filed Dec. 18, 2009
The Integrated Servo concept implements some or all major servo subfunctions for a storage device in Integrated Servo fields comprising sequences of encoded bits having selected mathematical properties. The Integrated Servo field is composed of a number of encoded sequences, which are members of a selected allowable sequence set that is constrained to provide some or all of the following functions: the Servo Track Mark (STM), the Position Error Signal (PES) and higher level positional information such as the track-ID. Thus, for example, an Integrated Servo embodiment would not need to have separate track ID fields using Gray code to encode the track ID. The integrated servo fields can provide a fractional Position Error Signal (PES) in relation to the center of a data track through the relative amplitude of the signal read for adjacent sequences disposed laterally across the tracks. The servo system detects the sequences in the signal from the read head using a set of digital filters corresponding to the set of encoded sequences. Embodiments of Integrated Servo constraint the placement of sequences so that only mathematically orthogonal sequences are placed next to each other on adjacent tracks. If the servo timing mark (STM) is implemented as part of the Integrated Servo it may or may not be detectable while seeking.
Augmented-servo-burst patterns in which information is encoded in addition to the fractional track PES have been described in the prior art. One example includes Gray code track ID fields plus diagonal burst PES with partial track ID information. See, for example, U.S. Pat. No. 7,110,209 to Ehrlich, et al. (Sep. 19, 2006).
U.S. Pat. No. 8,000,048 to Wilson (Aug. 16, 2011) describes use of phase-type servo patterns for track identification. The servo pattern include multiple circumferentially-spaced chevron patterns of discrete patterned servo islands. The chevron patterns are arranged to indicate the absolute radial position of the head without the need for separate track identification fields.
FIG. 3B illustrates the fields in a selected servo ID (SID) 20 according to the prior art. The preamble precedes Servo Address Mark (SAM) which is a timing mark which is used to synchronize data within the servo fields, and also provides timing information for write and read operations in the data portions of the disk. Second, the SID supplies a multi-bit digital field, which provides a coarse track-ID (TID) number and additional information to identify the physical SID number. The TID is typically written in Gray code as the presence or absence of recorded dibits. During seek operations, when the head is moving across tracks, the head can typically only read a portion of the Gray code in each TID. The Gray code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek.
The SID also supplies a position error field (A & B bursts in this example), which provides the fractional-track Position Error Signal (PES). Auxiliary functions, such as amplitude measurement or repeatable run-out (RRO) fields are sometimes also used. During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track. The typical PES patterns include either two or four bursts that are identical sets of high frequency magnetic flux transitions. FIG. 3B shows an example using only two PES bursts. The PES bursts are arranged in a pattern which generates a signal in the read head that is a function of the position of the read in relation to the centerline of the track. For example, the A and B bursts can be radially offset from each other by a half a track width and are sequential in the circumferential direction. Unlike the track-ID (TID) field number, the conventional PES bursts do not encode numerical information. The PES burst pattern is repeated for each set of two or four tracks, so only local information is provided.
The write-to-read gap 33 is included to allow for the physical separation between the write head 32 and the read head 33 in slider 31 and to provide the time/distance needed to switch from writing data to reading the next servo sector ID (SID) 20. (See FIG. 3A). The servo gate assertion period (window) 25A begins in the preamble and ends in this example with the RRO field. The bulk of the write-to-read gap is caused by the physical separation between the writer and reader. In most head designs the reader leads the writer as shown, so when the writer reaches the end of the data sector, the reader is already some distance beyond the end of the data sector which creates a physical gap. In addition some gap is needed to allow for the time needed for the drive's control systems to switch from writing to reading, but this switching gap is much smaller than the physical writer to reader separation. Accordingly servo systems have typically included a write to read gap 33 in the track format between the end of a writable data sector and the start of the following servo sector information.
A complicating factor in minimizing the needed gap is that the geometrical relationship (skew) between the heads and the track varies with the position of the mechanical actuator that move the slider with the heads in an arc across the disk surface. U.S. Pat. No. 7,551,379 to Yu, et al. (Jun. 23, 2009) describes a system in which the write element leads the read element in the tangential direction of rotation of the magnetic disk. The servo sector information is arranged such that information that is not needed for write operation is placed at the end of the servo sector. In this way, the servo read operation can be terminated sooner and the write operation can initiate sooner after going over the servo sector.